I have a group of four holes in a plate that I will control relative to each other using GD&T. Can I do so without the use of a datum? These four holes are in a plate and I don’t really care where the holes come relative to the plate edges, etc.

I threw together the attached drawing as an example. I could add datums to the plate edges and give very generous tolerances in a control frame, but that just seems to add datums for the fun of it.

Robin Davidson
MDS Aero Support Corp.

Plate question
Click on drawing for larger view

Hi Robin,

I reviewed your drawing and read your question. The short answer to your question is yes, you can specify a position tolerance on a hole pattern without a datum reference to control just the spacing and orientation of the holes relative to each other. However, this is not sufficient to completely define the part.

Either the holes need to be located from the edges of the part, or the part edges need to be located from the hole pattern.

Which dimensioning method you choose depends on how the part fits and functions in its assembly. If the surface that is identified as datum A is a mounting surface, then it should be referenced as the primary datum in the position tolerance feature control frame. Then it will control the perpendicularity of the holes to the mounting surface and their spacing to each other to insure proper fit to the mating part.

If the edges locate the part in the assembly, a common method would be to identify one edge as datum B and the other as datum C and reference them in a second position tolerance feature control frame as B secondary, and C tertiary. Which edge is B and which edge is C depends on the assembly sequence. The coordinate toleranced dimensions should be converted to basic dimensions, and an equivalent cylindrical positional tolerance is specified in the second feature control frame. Datum A is repeated in the second feature control frame as the primary datum, since you must first orient the part before locating on the edges. See option A.

Position tolerance hole pattern - Option A

If the holes locate the part and not the edges, a common method is to specify the hole pattern as datum B and reference it as a secondary datum at MMC in a profile tolerance control frame applied to the edges of the part. Profile applied all around can be used to control the length and width of the part as well. See option B.

Position tolerance hole pattern - Option B

A lot of people wonder about this. Thanks for submitting a good question.